when i climbed it 3 years ago at the end of August, we started at 2pm and walked leisurely up so we could take pics on the way up.. slept in a hut and continued shortly after midnight and reached the top way too early - 2am. I had a t-shirt, hoodie and a shell jacket on.(really thin but supposedly wind/water resistant) it was so bloody windy and cold at the top that I had to take my raincoat out and put it on for extra layer. It didnt work. The vending machine at the top sells warm drinks but it's not very warm. Luckily around 3am, the huts at the very top opened up and I ate a bowl of hot udon. Best damn udon I ever had. Once the sun comes up, regardless of wind, it won't be cold. And if you go down the Yoshida trail back to Kawaguchiko, it will be hell on earth if it is a sunny day.. The sun will beat down on you and it's all switchbacks at 45 degree angle? and loose volcanic material. Make sure you bring a mask with you. YOU WILL NEED IT.. and make sure you have water.. There aren't huts along the way after you go below 8th stage.. And be careful of your camera. the volcanic dust gets kicked up a lot and it might jam your camera. Mine got stuck and it would not close.. Have fun